For rigid, non-porous, and dense materials, achieving alignment between experimental normal incidence sound transmission loss (STL) values from impedance tube measurements under the ASTM E2611 standard and theoretical STL predicted by Mass Law required precise control of boundary conditions and mitigation of air leaks. Air leaks often originate from specimens cut slightly smaller than the impedance tube diameter, a common issue associated with cutting processes, which leads to distorted STL values. Conversely, edge constraint effects arise when specimens exceed the tube diameter, compromising the establishment of a free-free boundary condition essential for accurate measurements. The present work investigated normal incidence STL for commercially used polycarbonate, acrylic, and panels fabricated from recycled polypropylene. Initial attempts to improve boundary conditions, such as wrapping edges with Teflon tape or using 3-D-printed ring holders of different diameters, were insufficient, as they either introduced excessive edge stiffness or did not eliminate air leaks. Petroleum jelly was tested to address air leaks while preserving a free boundary; however, its high viscosity hindered complete sealing and caused STL discrepancies at higher frequencies. A significant improvement was achieved using a water-based lubricant, which effectively sealed without edge constraint, resulting in experimental STL values closely matching Mass Law predictions.
Mago et al. (Wed,) studied this question.